In typical code division multiple access (CDMA) systems, several users transmit over a common communication channel, typically using the direct sequence spread spectrum technique. The receiver in this environment receives a signal that is the sum of all of the transmitted signals in noise, and the receiver's job is to reliably decode the signal of interest from this received composite signal. The user signals are not synchronized in general and typically have unequal received signal strengths. The receiver may be a base station in a cellular telephone cell or personal communication network (PCN) cell, or it may be one of the user's receivers in a decentralized multiple access network.
An aspect of efficient multiuser receiver techniques for CDMA systems is to estimate the multiuser interference to the signal of interest from all the other users, and subtract it out, leaving a better estimate of the signal of interest. Performing such an operation requires an estimate of the data being sent by the other users, and of the cross-correlations between each other user and the user of interest.
Nonzero cross-correlations between the users arise due to the non-orthogonality of the signature sequences used by the various users. The magnitude of the cross-correlation between the users is determined by two factors: 1) the basic non-orthogonality of the spreading codes used; and 2) the frequency and phase offsets of the carriers at the receiver.
To perform normal coherent detection, the receiver must synchronize to the received symbol timing, carrier frequency and phase. In addition, the level of interference between the users is exaggerated if the interfering user is stronger then the user of interest. It is possible to use the local estimates of the synchronization parameters along with a knowledge of the set of signature sequences of the active users to estimate the set of inter-user cross-correlations. In addition, the energy of each user's signal may also be estimated at the receiver.
In order to implement the most effective multiuser receiver architectures for a CDMA system, it is necessary to have a knowledge of the set of cross-correlations between the signature sequences of the various users in the system. These cross-correlations are typically derived from a bank of N^2−N correlators, where N is the number of users active in the system. These correlators are usually implemented in dedicated, fabricated circuitry and similar hardware. As a result, if N is large, the number of correlators becomes excessive and tends to make such systems expensive.